The hydrogen separating capability of Pd alloy membranes is well known. Applications include hydrogenation and dehydrogenation reactions and recovery of hydrogen from petrochemical plant streams. Other applications are high temperature hydrogen separations, fuel cell power systems, hydrogen fueling stations, hydrocarbon reforming, and use in membrane reactors, devices that can simultaneously form a product and separate the reaction products.
There are several patented Pd and Pd/Ag alloy membrane devices and fabrication processes. All of these devices are poisoned severely by the presence of sulfur compounds, typically H2S, in a reducing environment. Solving the H2S poisoning problem is a necessity for application of metal membranes in the petroleum and petrochemical industries since all gas streams contain small amounts of H2S (0.01 to 1 ppm) which will poison metal membranes made from Pd and Pd/Ag alloys. The H2S poisoning problem will also be present for fuel cell power systems where hydrocarbons containing organic sulfur (such as gasoline, diesel fuel, and natural gas) are converted into synthesis gas.
Pd or Pd/Ag membrane materials useful for selectively separating hydrogen without poisoning the membrane have been actively pursued by the scientific community. A large base of technical literature exists specifically for Pd or Pd/Ag membranes and is the subject of numerous patents. Pd/Cu alloys and certain other Pd alloys are known to be much more resistant to H2S poisoning than Pd or Pd/Ag alloys.
There are two types of Pd and Pd alloy membranes. The first type is a Pd or Pd alloy foil membrane, a foil being a “free-standing” or unsupported membrane. The second type is a composite membrane that includes a substrate and a film of Pd or Pd—Ag that is supported on the substrate. With respect to the first type of membranes, McKinley is believed to have been the first to describe the beneficial properties of relatively thick palladium alloy foil membranes (25 to 100 microns in thickness). In U.S. Pat. No. 3,350,845, McKinley teaches the use of 0.1 mm thick (100 microns) Pd—Au alloy foil membranes for hydrogen separation which resist poisoning by sulfur compounds, including hydrogen sulfide. In U.S. Pat. No. 3,439,474, McKinley teaches the use of 25 micron thick 40 mass % Pd—Cu alloy foil membranes for the separation of hydrogen at elevated temperature and pressure. The primary benefits of the Pd—Cu alloy foil membrane include resistance to poisoning by H2S and higher hydrogen permeability compared to pure palladium.
With respect to the first type of membrane, U.S. Pat. No. 3,881,891 describes a method for increasing the hydrogen permeability of Pd alloy foil membranes and reducing the poisoning effects of sulfur compounds. The method involves adding water vapor to the gas mixture. They observed an increase in the hydrogen permeability in the presence of steam for all membranes tested. Exposure to water vapor also restored the hydrogen permeability of membranes previously exposed to H2S.
Also relating to the first type of membrane, U.S. Pat. No. 6,103,028 describes methods for reducing the thickness of palladium alloy (Pd—Cu, Pd—Ag, Pd—Ru, and Pd—Y) foil membranes to increase the hydrogen flux. The foil membranes are annealed at 320° C. under hydrogen and then etched by exposure to mineral acids or by electrochemically removing metal from the membranes.
Further relating to the first type of membranes, U.S. Pat. No. 6,152,995 describes a process to increase the flux of hydrogen through a metal foil membrane by chemical etching using a mineral acid such as HNO3 or mixtures of HNO3 and HCl. This patent also describes methods for finding leaks on metal foil membranes and techniques to repair such leaks.
The second type of membranes is comprised of a thin film of Pd deposited on a substrate that could be dense or porous. In this regard, U.S. Pat. No. 5,149,420 discloses a method for forming a thin, micron thick layer of pure Pd on supports of dense Group IV-B and V-B metals such as niobium, vanadium, or tantalum, or titanium. In operation, the palladium layer catalyzes the dissociation of hydrogen molecules to hydrogen atoms, which can subsequently permeate through the Pd layer and the dense metal support layer. A Pd coating on the other side of the membrane allows for the recombination of hydrogen atoms to form molecular hydrogen. It should be noted that the primary source of diffusional resistance in such a membrane is due to the metal support layer.
Further relating to the second type of membranes, U.S. Pat. Nos. 5,451,386 and 5,652,020 teach the preparation of supported Pd membranes for hydrogen separation by electroless plating of a dense layer of Pd that is 10 to 20 microns thick on porous ceramic supports. In these membranes, the primary source of resistance to hydrogen diffusion is the Pd layer, not the ceramic support.
Apparently, Kikuchi and co-workers have fabricated Pd—Cu alloy films on porous supports by chemical deposition of Pd and then Cu with subsequent annealing (500° C. for 12 hours).